In general, when a current pulse of large energy is supplied to a machining gap at a high frequency, the material removal rate in electric discharge machining is improved. Meanwhile, the current pulse is desirable to have a high current peak, a steep rising edge and a steep falling edge. A typical wire electrode is made from brass and has a diameter of between 0.07 mm and 0.30 mm. During the machining, a predetermined tension is applied to the wire electrode. When a large current suddenly flows through such a wire electrode, the wire electrode has a risk of breaking.
In order to avoid such a risk, a power supply system, which includes a first power supply circuit and a second power supply circuit, is known. The first power supply circuit applies a high voltage to a degree that induces the electric discharge to a machining gap, but limits the current supplied to the machining gap to be as small as possible. The first power supply circuit has a first DC (direct current) power supply and a current limiting resistor with a large resistance value. The second power supply circuit supplies a large current to the machining gap as soon as the electric discharge is generated by the first power supply circuit. The second power supply circuit includes a second DC power supply and a low inductance line connecting the second DC power supply to the machining gap. A resistance component and an inductance component are minimized in the second power supply circuit.
FIG. 8 illustrates a typical current pulse flowing in a machining gap. A part of current i1 is supplied from the first power supply circuit. The first power supply circuit supplies a current as small as possible to the machining gap. However, the current flows into the machining gap from an unintentional floating capacitor that is generated on the low inductance line in the second power supply circuit. This current forms a part of current i1. A current i2 is supplied from the second power supply circuit. It is known that the wire electrode made from brass is melted and copper is precipitated when the current pulse in FIG. 8 is supplied to the machining gap. As the copper is attached to the work piece, the material removal rate of the work piece is significantly decreased. The larger the current i1 is, the more the copper is attached to the work piece.
When a voltage is applied to the machining gap from the DC power supply of the first power supply circuit, a rise in the voltage becomes slow due to the floating capacitor in the low inductance line. In other words, the rising edge of the voltage becomes blunt. Therefore, a delay time from applying the voltage in the machining gap for generating the electric discharge becomes longer. At this point, the material removal rate of the work piece significantly decreases.
The patent document 1 disclosed a wire electric discharge machining apparatus in which a power supply circuit for rough machining is connected to a machining gap through a low inductance line and a power supply circuit for finish machining is connected to the machining gap through a low capacitance line. A blocking diode is disposed between the inductance line and the machining gap. The blocking diode prevents a current from flowing from a floating capacitor in the inductance line to the machining gap.
The patent document 2 disclosed a power supply apparatus for an electric discharge machine in which a main power supply is connected to a machining gap through a low inductance line and an auxiliary power supply is connected to the machining gap through the low capacitance line. A blocking diode is disposed between the inductance line and the machining gap, and a reverse bias power supply is series-connected to the blocking diode. When rough machining is performed, a large current is supplied to the machining gap from the main power supply immediately after the electric discharge is generated by a voltage of the auxiliary power supply. When the finish machining is performed, a small current is supplied to the machining gap from the auxiliary power supply. It is mentioned that the blocking diode and the reverse bias power supply considerably decrease an effect of the floating capacitor in the low inductance line when the finish machining is performed.